Biological Clocks

If you have ever flown across several time zones, you have experienced jet lag. You arrived in a new time zone, but your body was still living on the time in the old zone. You were wide awake and ready for dinner in the middle of the night, and you wanted to sleep all day.

People suffer from jet lag because all living things have a biological clock. Plants and animals are all in rhythm with the natural divisions of time—day and night and the seasons.

At sunrise, plants open their leaves and begin producing food. At night, they rest. In the temperate zones of the earth, trees lose their leaves in fall as the days grow shorter, and there is less sunlight. In the spring, leaves and flowers begin growing again as the days lengthen.

Rain sets the rhythm of desert plants. Plants in the desert may appear dead for months or even years, but when it begins to rain, the plants seem to come to life overnight. The leaves turn green, and flowers appear. The plants produce seeds quickly, before the rain stops. These seeds may lie on the ground for years before the rain starts the cycle of growth again. The plants' biological clock gave the signal for these things to happen.

At dawn most birds wake up and start singing. When the sun goes down, they go to sleep. When spring arrives, they start looking for a mate. When winter comes, some birds fly to a region with a warmer climate. Their biological clock tells them it is time to do all these things.

Animals that live near the sea and depend on both the land and water for their food have their biological clocks set with the tides. When the tide goes out, they know it is time to search for the food that the sea left behind it.

Some insects seem to set their alarm clock to wake up at night. They are out all night looking for food and then sleep during the day. Honeybees have a very strong sense of time. They can tell by the position of the sun exactly when their favorite flowers open.

Some French scientists did an experiment with honeybees. They put out sugar water every morning at 10:00 and at noon, and the bees came to drink the water at exactly the right time. Then the scientists put the sugar water in a room that was brightly lit 24 hours a day. They started putting the sugar water out at 8:00 p.m. It took the bees a week to find it at the different hour, but from then on they came to eat in the evening instead of in the morning.

Later the scientists took the honeybees to New York. The bees came for the food at the time their bodies told them, only it was 3:00 p.m. New York time. Their bodies were still on Paris time.

Humans, like other animals, have a biological clock that tells us when to sleep and eat. It causes other changes too. Blood pressure is lower at night, the heartbeat is slower, and the body temperature is a little lower. We even go through several levels of sleep, cycles of deep and light sleep.

Other events occur in cycles too. More babies are born between midnight and dawn than at any other time. More natural deaths occur at night, but more heart attacks happen early in the morning. Most deaths from diseases in hospitals occur between midnight and 6 a.m. Some police say there are more violent crimes and traffic accidents when there is a full moon.

The honeybees in the experiment reset their biological clock for different feeding hours. Humans do this too. People who work at night learn to sleep during the day and eat at night. Students who fly halfway across the world to study in another country get used to the new time zone after a few days. When they go home, they change back again. Our bodies are controlled by a biological clock, but we can learn to reset it at a different time.